Primary hyperparathyroidism masked by antituberculous therapy-induced vitamin D deficiency

Antituberculous chemotherapy agents, particularly rifampicin and isoniazid, affect vitamin D metabolism and can create biochemical evidence of vitamin D deficiency. Vitamin D deficiency induces a state of resistance to parathyroid hormone. This study sought to explain the temporary resolution of hypercalcaemia and hypercalciuria, during antituberculous chemotherapy with rifampicin and isoniazid, in a subject with a surgically proven parathyroid adenoma and coincidental spinal tuberculosis. Serum ionized calcium, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, plasma parathyroid hormone, and 24-hour urine excretions of calcium, inorganic phosphorus and hydroxyproline were sequentially measured over a 3-year interval that included 18 months of antituberculous chemotherapy. Initial serum ionized calcium was 1.52 mmol/l (normal 1.20-1.35 mmol/l), 24-hour urine calcium excretion was 9.40 mmol/day (normal 1.25 to 7.50 mmol/day) and plasma intact PTH was 9.2 pmol/l (normal 0.0-4.5 pmol/l). During antituberculous chemotherapy the serum ionized calcium and 24-hour urine calcium excretion were normal but the plasma PTH rose to higher levels. Following completion of the chemotherapy, hypercalcaemia and hypercalciuria returned with levels similar to those observed pretreatment. Serum 25-hydroxyvitamin D was low at 6.25 nmol/l (normal 20 to 90 nmol/l) during antituberculous chemotherapy, but was normal before and after. Serum 1,25-dihydroxyvitamin D was normal throughout the 3-year interval. We conclude that the antituberculous chemotherapy induced relative vitamin D deficiency and resistance to parathyroid hormone action, thereby masking the hyperparathyroidism and hypercalcaemia until the chemotherapy was completed.